Generally, in an electrophotographic image forming method, printed materials are prepared via the steps set forth below. First, a photoreceptor is exposed to light to form a latent image on the photoreceptor and then, a toner is supplied onto the photoreceptor to develop the latent image to form a toner image. Subsequently, the toner image on the photoreceptor is transferred to a transfer material such as paper and the transferred image is subjected to heating or pressure to fix the toner image, whereby a printed material is prepared. Further, after transfer of the toner image, a toner remained on the photoreceptor is removed by a cleaning device, rendering it feasible to perform subsequent image formation.
Recently, full-color print making by using plural kinds of color toners has been conducted and to perform efficient preparation of full-color prints, however, speedup image formation has been sought. To achieve high-speed print making, toners have been required to achieve faster electric-charging capability and rapid fixability. Further, from the view-point of enhancement of fixability, reduction of consumed energy in image formation is required from the consciousness of the global environment. Accordingly, there has been noted development of a toner corresponding to a technique of so-called low-temperature fixing.
A toner image formed on transfer paper is required to be melted in a state exhibiting a certain extent of viscosity under a prescribed condition and strong adhesion to the transfer paper. In cases when only the toner on the image surface is melted and the toner on the transfer paper side is only softened while the toner image passes through a fixing device, the toner which has been transferred to a transfer paper does not completely melt and does not achieve sufficient adhesion onto a transfer paper. Consequently, a toner image on the transfer paper adheres to a heating roller via a melted toner, causing image staining, so-called cold offset. Alternatively when a toner melts to such an extent that the viscosity of the toner is greatly reduced, a melted toner image ruptures and is transferred onto both the transfer paper and the fixing roller, causing image staining, so-called hot offset offset.
Thus, to achieve high-speed print making and low temperature fixability, a toner is required perform melting in a state exhibiting a certain extent of viscosity and strong adhesion onto transfer paper, so that offset resistance performance to inhibit occurrence of image staining due to melting troubles of a toner is also desired. Accordingly, a physical property of a binder resin with respect to heat has become one of the important factors affecting offset resistance. Further, such a physical property of a binder resin with respect to heat is one of important factors to achieve low temperature fixing.
Thus, a toner capable of achieving both low temperature fixability and offset resistance has been desired and there has been studied designing a toner to resolve this problem with noting a binder resin constituting such a toner. Examples thereof include control of a low molecular weight component and a high molecular weight component in the binder resin and introduction of a crosslinking structure. Specifically, there was disclosed a technique of a toner employing a binder resin obtained by use of a styrene-acrylic acid copolymeric resin having a broad molecular weight distribution without a high molecular weight region and a metal compound in which a crosslinking structure was formed between carboxyl groups of the polymer and the metal compound (as described in JP 61-110156A). This technique intended to achieve enhanced offset resistance substantially by an increased molecular weight of a binder resin through formation of a crosslinking structure, however, an increased amount of the metal compound caused a catalytic action, resulting in gelation of the resin and leading to inhibition of fixing.
There was also studied designation of a toner corresponding to low temperature fixability by controlling the acid value, the hydroxyl group value and the molecular weight distribution of a polyester resin, and components insoluble in tetrahydrofuran as described in JP 9-204071). However, it was proved that this technique resulted in lowering of the melting temperature, leading to reduced offset resistance.
Thus, noting a binder resin as a toner constituent, studies of a toner capable of achieving both low temperature fixability and offset resistance have been made but further investigation is required.